Although most electronic devices operate from power supplied at a more-or-less constant voltage (referred to as direct current or DC), many devices presenting larger loads such as appliances or other devices having motors or heating elements are designed to operate on power supplied at a sinusoidally varying voltage (referred to as alternating current or AC). In most of the world, power is generated and transmitted as AC power since the sinusoidally varying voltage can be easily changed using transformers and reduced transmission infrastructure costs and reduced transmission losses can be achieved by using high voltage for transmission and reducing the voltage at a location proximate to the load. AC power is also convenient to generate where the availability of an energy source is substantially constant, for example in fossil fuel, nuclear and hydroelectric powered generators.
However, so-called renewable energy sources such as solar collectors and wind power have become of substantial interest in recent years in order to conserve fossil and nuclear fuels and to avoid environmental pollution and/or reduce the likelihood of accidents and to reduce the need for additional hydroelectric generation facilities which carry a very high initial cost. Renewable energy sources are, by their nature, only intermittently available with highly variable energy delivery and, therefore, generally require some form of power storage as charge in a battery or capacitor bank at a DC voltage which may vary in magnitude with the amount of power stored. Such storage also necessarily requires conversion to AC power if power is to be transferred more than a short distance or coupled to an AC power distribution grid. Therefore a DC to AC power converter capable of operating at very high voltage is generally required.
So-called modular multi-level converters (MMCs) have been increasingly considered for high and medium voltage variable frequency applications since a modular construction facilitates adaptation of a single converter module design to a wide variety and scale of applications such as, for example, interfacing with a power distribution grid or controlling and powering AC motors which must be operated at variable speed by simply assembling and interconnecting modules in accordance with power delivery requirements. However, in normal MMC operation, the capacitors in the respective modules must buffer power fluctuations at line frequency and second order harmonic of the line frequency. This requirement inherently results in a requirement for high value and large size of the module capacitors especially where output line frequency must be variable since capacitor voltage ripple will increase with decreasing frequency and, at a frequency of zero Hz (e.g. DC), becomes infinite. This latter fact is a major issue for AC motor starting where the frequency must increase from zero Hz, especially where high starting torque is required.
It has been proposed to reduce the capacitor energy ripple by shifting the arm (e.g. the portion of the circuit supplying positive or negative half of a given phase of a multi-phase arrangement) currents toward a higher frequency to reduce the capacitor energy ripple by injecting a high frequency sinusoidal circulating (e.g. transferring energy between a capacitor and inductor) current on the respective phases during low frequency operation. However, such an approach increases stress on the switches of the modules and requires converters to be de-rated for low frequency operation while no reduction in capacitor size or value is achieved; limiting the potential power density of the converter modules.